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#REDIRECT [[C++#Language]]
{{Short description|Set of rules defining correctly structured C++ program}}
[[File:Orwell Dev-Cpp zh cn.jpg|thumb|300px|A snippet of C++ code]]

The '''syntax of C++''' is [[syntax|the set of rules]] defining how a [[C++]] program is written and compiled.

C++ syntax is largely inherited from the syntax of its ancestor language [[C (programming language)|C]], and has influenced the syntax of several later languages including but not limited to [[Java (programming language)|Java]], [[C Sharp (programming language)|C#]], and [[Rust (programming language)|Rust]].

== Basics ==
Much of C++'s syntax aligns with [[C syntax]], as C++ provides backwards compatibility with C.

=== Identifier ===
An [[Identifier#In computer languages|identifier]] is the name of an element in the [[source code|code]]. There are certain standard [[Naming conventions (programming)|naming conventions]] to follow when selecting names for elements. Identifiers in C++ are [[Case sensitivity|case-sensitive]].

An identifier can contain:
* Any Unicode character that is a letter (including numeric letters like [[Roman numerals]]) or digit.
* [[Currency sign]] (such as ¥).
* Connecting punctuation character (such as [[Underscore|_]]).

An identifier cannot:
* Start with a digit.
* Be equal to a reserved keyword, null literal or [[Boolean data type|Boolean]] literal.

The identifier <code>nullptr</code> is not a reserved word, but is a global constant that refers to a [[null pointer]] literal.

=== Keywords ===
The following words may not be used as identifier names or redefined.<ref name=cppreferencekeywords />
{{div col|colwidth=15em}}
* <code>alignas</code>
* <code>alignof</code>
* <code>and</code>
* <code>and_eq</code>
* <code>asm</code>
* <code>auto</code>
* <code>bitand</code>
* <code>bitor</code>
* <code>bool</code>
* <code>break</code>
* <code>case</code>
* <code>catch</code>
* <code>char</code>
* <code>char8_t</code>
* <code>char16_t</code>
* <code>char32_t</code>
* <code>class</code>
* <code>compl</code>
* <code>concept</code>
* <code>const</code>
* <code>consteval</code>
* <code>constexpr</code>
* <code>constinit</code>
* <code>const_cast</code>
* <code>continue</code>
* <code>contract_assert</code>
* <code>co_await</code>
* <code>co_return</code>
* <code>co_yield</code>
* <code>decltype</code>
* <code>default</code>
* <code>default</code>
* <code>do</code>
* <code>double</code>
* <code>dynamic_cast</code>
* <code>else</code>
* <code>enum</code>
* <code>explicit</code>
* <code>export</code>
* <code>extern</code>
* <code>false</code>
* <code>float</code>
* <code>for</code>
* <code>friend</code>
* <code>goto</code>
* <code>if</code>
* <code>import</code>
* <code>inline</code>
* <code>int</code>
* <code>long</code>
* <code>module</code>
* <code>mutable</code>
* <code>namespace</code>
* <code>new</code>
* <code>noexcept</code>
* <code>not</code>
* <code>not_eq</code>
* <code>nullptr</code>
* <code>operator</code>
* <code>or</code>
* <code>or_eq</code>
* <code>private</code>
* <code>protected</code>
* <code>public</code>
* <code>register</code>
* <code>reinterpret_cast</code>
* <code>requires</code>
* <code>return</code>
* <code>short</code>
* <code>signed</code>
* <code>sizeof</code>
* <code>static</code>
* <code>static_assert</code>
* <code>static_cast</code>
* <code>struct</code>
* <code>switch</code>
* <code>template</code>
* <code>this</code>
* <code>thread_local</code>
* <code>throw</code>
* <code>true</code>
* <code>try</code>
* <code>typedef</code>
* <code>typeid</code>
* <code>typename</code>
* <code>union</code>
* <code>unsigned</code>
* <code>using</code>
* <code>virtual</code>
* <code>void</code>
* <code>volatile</code>
* <code>wchar_t</code>
* <code>while</code>
* <code>xor</code>
* <code>xor_eq</code>
{{div col end}}

=== Identifiers with special meaning ===
The following words may be used as identifier names, but bear special meanings in certain contexts.
{{div col|colwidth=15em}}
* <code>final</code>
* <code>override</code>
* <code>pre</code>
* <code>post</code>
* <code>trivially_relocatable_if_eligible</code>
* <code>replaceable_if_eligible</code>
{{div col end}}

=== Preprocessor directives ===
The following tokens are recognised by the [[C preprocessor|preprocessor]] in the context of preprocessor directives.
{{div col|colwidth=15em}}
* <code>#if</code>
* <code>#elif</code>
* <code>#else</code>
* <code>#endif</code>
* <code>#ifdef</code>
* <code>#ifndef</code>
* <code>#elifdef</code>
* <code>#elifndef</code>
* <code>#define</code>
* <code>#undef</code>
* <code>#include</code>
* <code>#embed</code>
* <code>#line</code>
* <code>#error</code>
* <code>#warning</code>
* <code>#pragma</code>
* <code>#defined</code>
* <code>#__has_include</code>
* <code>#__has_cpp_attribute</code>
* <code>#__has_embed</code>
{{div col end}}

=== Code blocks ===
The separators {{mono|{{(}}}} and {{mono|{{)}}}} signify a code block and a new scope. Class members and the body of a [[Method (computer programming)|method]] are examples of what can live inside these braces in various contexts.

Inside of method bodies, braces may be used to create new scopes, as follows:

<syntaxhighlight lang="cpp">
void doSomething() {
int a;

{
int b;
a = 1;
}

a = 2;
b = 3; // Illegal because the variable b is declared in an inner scope.
}
</syntaxhighlight>

=== Comments ===
Java has two kinds of [[Comment (computer programming)|comments]]: ''traditional comments'' and ''end-of-line comments''.

Traditional comments, also known as block comments, start with <code>/*</code> and end with <code>*/</code>, they may span across multiple lines.

<syntaxhighlight lang="cpp">
/* This is a multi-line comment.
It may occupy more than one line. */
</syntaxhighlight>

End-of-line comments start with <code>//</code> and extend to the end of the current line.
<syntaxhighlight lang="cpp">
// This is an end-of-line comment
</syntaxhighlight>

Documentation comments in the source files are processed by the external [[Doxygen]] tool to generate documentation. This type of comment is identical to traditional comments, except it starts with <code>/**</code> and follows conventions defined by the Doxygen tool. Technically, these comments are a special kind of traditional comment and they are not specifically defined in the language specification.
<syntaxhighlight lang="cpp">
/**
* This is a documentation comment.
*
* @author John Doe
*/
</syntaxhighlight>

=== Command-line arguments ===
Much like in C, the [[parameter]]s given on a [[command line]] are passed to a C++ program with two predefined variables - the count of the command-line arguments in {{code|argc}} and the individual [[Parameter|arguments]] as [[character string]]s in the pointer array {{code|argv}}. So the command:

myFilt p1 p2 p3

results in something like:
{|class="wikitable" style="font-family: monospace,monospace;"
|-
|m||y||F||i||l||t||style="background:#CCC;"|\0||p||1||style="background:#CCC;"|\0||p||2||style="background:#CCC;"|\0||p||3||style="background:#CCC;"|\0
|-
|colspan="7" align="center"|argv[0]||colspan="3" align="center"|argv[1]||colspan="3" align="center"|argv[2]||colspan="3" align="center"|argv[3]
|}

While individual strings are arrays of contiguous characters, there is no guarantee that the strings are stored as a contiguous group.

The name of the program, {{code|argv[0]}}, may be useful when printing diagnostic messages or for making one binary serve multiple purposes. The individual values of the parameters may be accessed with {{code|argv[1]}}, {{code|argv[2]}}, and {{code|argv[3]}}, as shown in the following program:

<syntaxhighlight lang=cpp>
import std;

int main(int argc, char* argv[]) {
std::println("{}", argc);
for (size_t i = 0; i < argc; ++i)
std::println("argv[{}] = {}", i, argv[i]);
}
</syntaxhighlight>

== Objects ==
{{Main|C++ classes}}
C++ introduces [[object-oriented programming]] (OOP) features to C. It offers [[class (computer science)|class]]es, which provide the four features commonly present in OOP (and some non-OOP) languages: [[Abstraction (computer science)|abstraction]], [[Information hiding|encapsulation]], [[Inheritance (object-oriented programming)|inheritance]], and [[Polymorphism (computer science)|polymorphism]]. One distinguishing feature of {{nowrap|C++}} classes compared to classes in other programming languages is support for deterministic [[destructor (computer science)|destructors]], which in turn provide support for the [[Resource Acquisition is Initialization]] (RAII) concept.

== Object storage ==
As in C, C++ supports four types of [[memory management]]: static storage duration objects, thread storage duration objects, automatic storage duration objects, and dynamic storage duration objects.<ref name="C++11 3.7">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]. ''[https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7 Storage duration [basic.stc]''</ref>

=== Static storage duration objects ===
Static storage duration objects are created before <code>main()</code> is entered (see exceptions below) and destroyed in reverse order of creation after <code>main()</code> exits. The exact order of creation is not specified by the standard (though there are some rules defined below) to allow implementations some freedom in how to organize their implementation. More formally, objects of this type have a lifespan that "shall last for the duration of the program".<ref name="C++11 3.7.1">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]. ''[https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.1 Static Storage duration [basic.stc.static]''</ref>

Static storage duration objects are initialized in two phases. First, "static initialization" is performed, and only ''after'' all static initialization is performed, "dynamic initialization" is performed. In static initialization, all objects are first initialized with zeros; after that, all objects that have a constant initialization phase are initialized with the constant expression (i.e. variables initialized with a literal or <code>constexpr</code>). Though it is not specified in the standard, the static initialization phase can be completed at compile time and saved in the data partition of the executable. Dynamic initialization involves all object initialization done via a constructor or function call (unless the function is marked with <code>constexpr</code>, in C++11). The dynamic initialization order is defined as the order of declaration within the compilation unit (i.e. the same file). No guarantees are provided about the order of initialization between compilation units.

=== Thread storage duration objects ===
Variables of this type are very similar to static storage duration objects. The main difference is the creation time is just before thread creation, and destruction is done after the thread has been joined.<ref name="C++11 3.7.2">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]. ''[https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018}} §3.7.2 Thread Storage duration [basic.stc.thread]''</ref>

=== Automatic storage duration objects ===
The most common variable types in C++ are [[local variable]]s inside a [[Function (computer programming)|function]] or block, and temporary variables.<ref name="C++11 3.7.3">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]. ''[https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.3 Automatic Storage duration [basic.stc.auto]''</ref> The common feature about automatic variables is that they have a lifetime that is limited to the scope of the variable. They are created and potentially initialized at the point of declaration (see below for details) and destroyed in the ''reverse'' order of creation when the scope is left. This is implemented by allocation on the [[Stack-based memory allocation|stack]].

Local variables are created as the point of execution passes the declaration point. If the variable has a constructor or initializer this is used to define the initial state of the object. Local variables are destroyed when the local block or function that they are declared in is closed. C++ destructors for local variables are called at the end of the object lifetime, allowing a discipline for automatic resource management termed [[Resource Acquisition Is Initialization|RAII]], which is widely used in C++.

Member variables are created when the parent object is created. Array members are initialized from 0 to the last member of the array in order. Member variables are destroyed when the parent object is destroyed in the reverse order of creation. i.e. If the parent is an "automatic object" then it will be destroyed when it goes out of scope which triggers the destruction of all its members.

Temporary variables are created as the result of expression evaluation and are destroyed when the statement containing the expression has been fully evaluated (usually at the <code>;</code> at the end of a statement).

=== Dynamic storage duration objects ===
{{Main|new and delete (C++)}}

These objects have a dynamic lifespan and can be created directly with a call to {{cpp|new}} and destroyed explicitly with a call to {{cpp|delete}}.<ref name="C++11 3.7.4">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]. ''[https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf Programming Languages – C++11 Draft (n3797)] {{Webarchive|url=https://web.archive.org/web/20181002093659/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf |date=2 October 2018 }} §3.7.4 Dynamic Storage duration <nowiki>[</nowiki>basic.stc.dynamic<nowiki>]</nowiki>''</ref> C++ also supports <code>malloc</code> and <code>free</code>, from C, but these are not compatible with {{cpp|new}} and {{cpp|delete}}. Use of {{cpp|new}} returns an address to the allocated memory. The C++ Core Guidelines advise against using {{cpp|new}} directly for creating dynamic objects in favor of smart pointers through {{cpp|make_unique<T>}} for single ownership and {{cpp|make_shared<T>}} for reference-counted multiple ownership,<ref>{{Cite web |url=https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#r11-avoid-calling-new-and-delete-explicitly |title=C++ Core Guidelines |website=isocpp.github.io |access-date=2020-02-09 |archive-date=8 February 2020 |archive-url=https://web.archive.org/web/20200208160101/http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#r11-avoid-calling-new-and-delete-explicitly |url-status=live}}</ref> which were introduced in C++11.

== Encapsulation ==
[[Information hiding|Encapsulation]] is the hiding of information to ensure that data structures and operators are used as intended and to make the usage model more obvious to the developer. C++ provides the ability to define classes and functions as its primary encapsulation mechanisms. Within a class, members can be declared as either public, protected, or private to explicitly enforce encapsulation. A public member of the class is accessible to any function. A private member is accessible only to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member is accessible to members of classes that inherit from the class in addition to the class itself and any friends.

The object-oriented principle ensures the encapsulation of all and only the functions that access the internal representation of a type. C++ supports this principle via member functions and friend functions, but it does not enforce it. Programmers can declare parts or all of the representation of a type to be public, and they are allowed to make public entities not part of the representation of a type. Therefore, C++ supports not just object-oriented programming, but other decomposition paradigms such as [[Modularity (programming)|modular programming]].

It is generally considered good practice to make all [[data]] private or protected, and to make public only those functions that are part of a minimal interface for users of the class. This can hide the details of data implementation, allowing the designer to later fundamentally change the implementation without changing the interface in any way.<ref name="cppcs">{{Cite book |first1=Herb |last1=Sutter |first2=Andrei |last2=Alexandrescu |author-link1=Herb Sutter |author-link2=Andrei Alexandrescu |year=2004 |title=C++ Coding Standards: 101 Rules, Guidelines, and Best Practices |publisher=Addison-Wesley}}</ref><ref name="industrialcpp">{{Cite book |last1=Henricson |first1=Mats |last2=Nyquist |first2=Erik |title=Industrial Strength C++ |publisher=Prentice Hall |year=1997 |isbn=0-13-120965-5 |url=https://archive.org/details/industrialstreng0000henr}}</ref>

=== Inheritance ===
[[Inheritance (object-oriented programming)|Inheritance]] allows one data type to acquire properties of other data types. Inheritance from a [[base class]] may be declared as public, protected, or private. This access specifier determines whether unrelated and derived classes can access the inherited public and protected members of the base class. Only public inheritance corresponds to what is usually meant by "inheritance". The other two forms are much less frequently used. If the access specifier is omitted, a "class" inherits privately, while a "struct" inherits publicly. Base classes may be declared as virtual; this is called [[virtual inheritance]]. Virtual inheritance ensures that only one instance of a base class exists in the inheritance graph, avoiding some of the ambiguity problems of multiple inheritance.

[[Multiple inheritance]] is a C++ feature allowing a class to be derived from more than one base class; this allows for more elaborate inheritance relationships. For example, a "Flying Cat" class can inherit from both "Cat" and "Flying Mammal". Some other languages, such as [[C Sharp (programming language)|C#]] or [[Java (programming language)|Java]], accomplish something similar (although more limited) by allowing inheritance of multiple [[Interface (object-oriented programming)|interfaces]] while restricting the number of base classes to one (interfaces, unlike classes, provide only declarations of member functions, no implementation or member data). An interface as in C# and Java can be defined in {{nowrap|C++}} as a class containing only pure virtual functions, often known as an [[abstract base class]] or "ABC". The member functions of such an abstract base class are normally explicitly defined in the derived class, not inherited implicitly. C++ virtual inheritance exhibits an ambiguity resolution feature called [[Dominance (C++)|dominance]].

== Operators and operator overloading ==
{| class="wikitable plainrowheaders floatright"
|+ Operators that cannot be overloaded
! Operator
! Symbol
|-
| Scope resolution
| {{cpp| ::}}
|-
| Conditional
| {{cpp| ?:}}
|-
| dot
| {{cpp| .}}
|-
| Member selection
| {{cpp| .*}}
|-
| "[[sizeof]]"
| {{cpp| sizeof}}
|-
| "[[typeid]]"
| {{cpp| typeid}}
|}

{{Main|Operators in C and C++}}

C++ provides more than 35 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can be [[Operator overloading|overloaded]] for user-defined types, with a few notable exceptions such as member access (<code>.</code> and <code>.*</code>) and the conditional operator. The rich set of overloadable operators is central to making user-defined types in C++ seem like built-in types.

Overloadable operators are also an essential part of many advanced C++ programming techniques, such as [[smart pointer]]s. Overloading an operator does not change the precedence of calculations involving the operator, nor does it change the number of operands that the operator uses (any operand may however be ignored by the operator, though it will be evaluated prior to execution). Overloaded "<code>&&</code>" and "<code>||</code>" operators lose their [[short-circuit evaluation]] property.

== Polymorphism ==
{{See also|Polymorphism (computer science)}}

[[Polymorphism (computer science)|Polymorphism]] enables one common interface for many implementations, and for objects to act differently under different circumstances.

C++ supports several kinds of ''static'' (resolved at [[compile-time]]) and ''dynamic'' (resolved at [[Execution (computing)#Runtime|run-time]]) [[Polymorphism (computer science)|polymorphisms]], supported by the language features described above. [[Compile-time polymorphism]] does not allow for certain run-time decisions, while [[runtime polymorphism]] typically incurs a performance penalty.

=== Dynamic polymorphism ===

==== Inheritance ====
{{See also|Subtyping}}

Variable pointers and references to a base class type in C++ can also refer to objects of any derived classes of that type. This allows arrays and other kinds of containers to hold pointers to objects of differing types (references cannot be directly held in containers). This enables dynamic (run-time) polymorphism, where the referred objects can behave differently, depending on their (actual, derived) types.

C++ also provides the <syntaxhighlight lang="C++" inline>dynamic_cast</syntaxhighlight> operator, which allows code to safely attempt conversion of an object, via a base reference/pointer, to a more derived type: ''downcasting''. The ''attempt'' is necessary as often one does not know which derived type is referenced. (''Upcasting'', conversion to a more general type, can always be checked/performed at compile-time via <syntaxhighlight lang="C++" inline>static_cast</syntaxhighlight>, as ancestral classes are specified in the derived class's interface, visible to all callers.) <syntaxhighlight lang="C++" inline>dynamic_cast</syntaxhighlight> relies on [[run-time type information]] (RTTI), metadata in the program that enables differentiating types and their relationships. If a <syntaxhighlight lang="C++" inline>dynamic_cast</syntaxhighlight> to a pointer fails, the result is the <syntaxhighlight lang="C++" inline>nullptr</syntaxhighlight> constant, whereas if the destination is a reference (which cannot be null), the cast throws an exception. Objects ''known'' to be of a certain derived type can be cast to that with <syntaxhighlight lang="C++" inline>static_cast</syntaxhighlight>, bypassing RTTI and the safe runtime type-checking of <syntaxhighlight lang="C++" inline>dynamic_cast</syntaxhighlight>, so this should be used only if the programmer is very confident the cast is, and will always be, valid.

==== Virtual member functions ====
Ordinarily, when a function in a derived class [[Method overriding (programming)|overrides]] a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference in the number or type of parameters between two or more definitions of that function. Hence, at compile time, it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is called [[dynamic dispatch]]. [[virtual functions|Virtual member functions]] or ''methods''<ref>{{Cite book |quote=A virtual member function is sometimes called a ''method''. |first=Bjarne |last=Stroustrup |year=2000 |page=310 |title=The C++ Programming Language |edition=Special |publisher=Addison-Wesley |isbn=0-201-70073-5}}</ref> allow the most specific implementation of the function to be called, according to the actual run-time type of the object. In C++ implementations, this is commonly done using [[virtual function table]]s. If the object type is known, this may be bypassed by prepending a [[fully qualified name|fully qualified class name]] before the function call, but in general calls to virtual functions are resolved at run time.

In addition to standard member functions, operator overloads and destructors can be virtual. An inexact rule based on practical experience states that if any function in the class is virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extension copy constructors, cannot be virtual. Nonetheless, a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case, a common solution is to create a <syntaxhighlight lang="C++" inline>clone()</syntaxhighlight> (or similar) virtual function that creates and returns a copy of the derived class when called.

A member function can also be made "pure virtual" by appending it with <syntaxhighlight lang="C++" inline>= 0</syntaxhighlight> after the closing parenthesis and before the semicolon. A class containing a pure virtual function is called an ''abstract class''. Objects cannot be created from an abstract class; they can only be derived from. Any derived class inherits the virtual function as pure and must provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created. A program that attempts to create an object of a class with a pure virtual member function or inherited pure virtual member function is ill-formed.

== Static polymorphism ==
{{See also|Parametric polymorphism|ad hoc polymorphism}}

[[Function overloading]] allows programs to declare multiple functions having the same name but with different arguments (i.e. [[ad hoc polymorphism|''ad hoc'' polymorphism]]). The functions are distinguished by the number or types of their [[Parameter (computer science)|formal parameter]]s. Thus, the same function name can refer to different functions depending on the context in which it is used. The type returned by the function is not used to distinguish overloaded functions and differing return types would result in a compile-time error message.

When declaring a function, a programmer can specify for one or more parameters a [[default arguments|default value]]. Doing so allows the parameters with defaults to optionally be omitted when the function is called, in which case the default arguments will be used. When a function is called with fewer arguments than there are declared parameters, explicit arguments are matched to parameters in left-to-right order, with any unmatched parameters at the end of the parameter list being assigned their default arguments. In many cases, specifying default arguments in a single function declaration is preferable to providing overloaded function definitions with different numbers of parameters.

=== Templates ===
{{main|Template (C++)}}
{{See also|Template metaprogramming|Generic programming}}
[[C++ templates]] enable [[generic programming]]. {{nowrap|C++}} supports function, class, alias, and variable templates. Templates may be parameterized by types, compile-time constants, and other templates. Templates are implemented by ''instantiation'' at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class instance. Some substitutions are not possible; these are eliminated by an overload resolution policy described by the phrase "[[Substitution failure is not an error]]" (SFINAE). Templates are a powerful tool that can be used for [[generic programming]], [[template metaprogramming]], and code optimization, but this power implies a cost. Template use may increase [[object code]] size, because each template instantiation produces a copy of the template code: one for each set of template arguments, however, this is the same or smaller amount of code that would be generated if the code were written by hand.<ref name=":0" /> This is in contrast to run-time generics seen in other languages (e.g., [[Generics in Java|Java]]) where at compile-time the type is erased and a single template body is preserved.

Templates are different from [[Macro (computer science)|macro]]s: while both of these compile-time language features enable conditional compilation, templates are not restricted to lexical substitution. Templates are aware of the semantics and type system of their companion language, as well as all compile-time type definitions, and can perform high-level operations including programmatic flow control based on evaluation of strictly type-checked parameters. Macros are capable of conditional control over compilation based on predetermined criteria, but cannot instantiate new types, recurse, or perform type evaluation and in effect are limited to pre-compilation text-substitution and text-inclusion/exclusion. In other words, macros can control compilation flow based on pre-defined symbols but cannot, unlike templates, independently instantiate new symbols. Templates are a tool for static [[Polymorphism (computer science)|polymorphism]] (see below) and [[generic programming]].

In addition, templates are a compile-time mechanism in C++ that is [[Turing-complete]], meaning that any computation expressible by a computer program can be computed, in some form, by a [[template metaprogramming|template metaprogram]] before runtime.

In summary, a template is a compile-time parameterized function or class written without knowledge of the specific arguments used to instantiate it. After instantiation, the resulting code is equivalent to code written specifically for the passed arguments. In this manner, templates provide a way to decouple generic, broadly applicable aspects of functions and classes (encoded in templates) from specific aspects (encoded in template parameters) without sacrificing performance due to abstraction.

Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code (i.e. [[parametric polymorphism]]). In particular, through the [[curiously recurring template pattern]], it is possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual functions. Because C++ templates are type-aware and [[Turing-complete]], they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through [[template metaprogramming]]. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings.<ref name=":0">{{cite web |access-date=8 March 2010 |publisher=EmptyCrate Software. Travel. Stuff. |location=articles.emptycrate.com/ |title=Nobody Understands C++: Part 5: Template Code Bloat |date=6 May 2008 |url=https://articles.emptycrate.com/2008/05/06/nobody_understands_c_part_5_template_code_bloat.html |quote=On occasion you will read or hear someone talking about C++ templates causing code bloat. I was thinking about it the other day and thought to myself, "self, if the code does exactly the same thing then the compiled code cannot really be any bigger, can it?" [...] And what about compiled code size? Each were compiled with the command g++ <filename>.cpp -O3. Non-template version: 8140 bytes, template version: 8028 bytes! |archive-date=25 April 2016 |archive-url=https://web.archive.org/web/20160425105303/http://articles.emptycrate.com/2008/05/06/nobody_understands_c_part_5_template_code_bloat.html |url-status=live}}</ref>

== Lambda expressions ==
C++ provides support for [[anonymous function]]s, also known as [[Lambda calculus|lambda expressions]], with the following form:

<syntaxhighlight lang="cpp">
[capture](parameters) -> return_type { function_body }
</syntaxhighlight>

Since C++20, the keyword {{code|2=cpp|1=template}} is optional for template parameters of lambda expressions:

<syntaxhighlight lang="cpp">
[capture]<template_parameters>(parameters) -> return_type { function_body }
</syntaxhighlight>

If the lambda takes no parameters, and no return type or other specifiers are used, the () can be omitted; that is,

<syntaxhighlight lang="cpp">
[capture] { function_body }
</syntaxhighlight>

The return type of a lambda expression can be automatically inferred, if possible; e.g.:

<syntaxhighlight lang="cpp">
[](int x, int y) { return x + y; } // inferred
[](int x, int y) -> int { return x + y; } // explicit
</syntaxhighlight>

The <syntaxhighlight lang="C++" inline>[capture]</syntaxhighlight> list supports the definition of [[Closure (computer programming)|closures]]. Such lambda expressions are defined in the standard as [[syntactic sugar]] for an unnamed [[function object]].

== Exception handling==
Exception handling is used to communicate the existence of a runtime problem or error from where it was detected to where the issue can be handled.<ref>{{Cite web |url=http://www.cl.cam.ac.uk/teaching/1314/CandC++/lecture7.pdf |title=<nowiki>C and C++ Exceptions | Templates</nowiki> |date=2013 |access-date=30 August 2016 |website=Cambridge Computer Laboratory - Course Materials 2013-14 |last=Mycroft |first=Alan |archive-date=13 May 2016 |archive-url=https://web.archive.org/web/20160513074615/http://www.cl.cam.ac.uk/teaching/1314/CandC++/lecture7.pdf |url-status=live}}</ref> It permits this to be done in a uniform manner and separately from the main code, while detecting all errors.<ref name="exception_summary">{{Cite book |title=The C++ Programming Language |last=Stroustrup |first=Bjarne |publisher=Addison Wesley |year=2013 |isbn=9780321563842 |pages=345}}</ref> Should an error occur, an exception is thrown (raised), which is then caught by the nearest suitable exception handler. The exception causes the current scope to be exited, and also each outer scope (propagation) until a suitable handler is found, calling in turn the destructors of any objects in these exited scopes.<ref>{{Cite book |title=The C++ Programming Language |last=Stroustrup |first=Bjarne |publisher=Addison Wesley |year=2013 |isbn=9780321563842 |pages=363–365}}</ref> At the same time, an exception is presented as an object carrying the data about the detected problem.<ref>{{Cite book |title=The C++ Programming Language |last=Stroustrup |first=Bjarne |publisher=Addison Wesley |year=2013 |isbn=9780321563842 |pages=345, 363}}</ref>

Some C++ style guides, such as Google's,<ref>{{cite web |title=Google C++ Style Guide |url=https://google.github.io/styleguide/cppguide.html#Exceptions |access-date=25 June 2019 |archive-date=16 March 2019 |archive-url=https://web.archive.org/web/20190316065327/http://google.github.io/styleguide/cppguide.html#Exceptions |url-status=live}}</ref> LLVM's,<ref>{{cite web |title=LLVM Coding Standards |url=https://llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions |website=LLVM 9 documentation |access-date=25 June 2019 |archive-date=27 June 2019 |archive-url=https://web.archive.org/web/20190627023217/http://llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions |url-status=live}}</ref> and Qt's,<ref>{{cite web |title=Coding Conventions |url=https://wiki.qt.io/Coding_Conventions |website=Qt Wiki |access-date=26 June 2019 |archive-date=26 June 2019 |archive-url=https://web.archive.org/web/20190626231458/https://wiki.qt.io/Coding_Conventions |url-status=live}}</ref> forbid the usage of exceptions.

The exception-causing code is placed inside a <syntaxhighlight lang="C++" inline>try</syntaxhighlight> block. The exceptions are handled in separate <syntaxhighlight lang="C++" inline>catch</syntaxhighlight> blocks (the handlers); each <syntaxhighlight lang="C++" inline>try</syntaxhighlight> block can have multiple exception handlers, as it is visible in the example below.<ref>{{Cite book |title=The C++ Programming Language |last=Stroustrup |first=Bjarne |publisher=Addison Wesley |year=2013 |isbn=9780321563842 |pages=344, 370}}</ref>
<!--"#include <iostream.h> is deprecated"-->
<syntaxhighlight lang="cpp" line="1">
import std;

int main() {
try {
std::vector<int> vec{3, 4, 3, 1};
int i{vec.at(4)}; // Throws an exception, std::out_of_range (indexing for vec is from 0-3 not 1-4)
} catch (const std::out_of_range& e) {
// An exception handler, catches std::out_of_range, which is thrown by vec.at(4)
std::println(stderr, "Accessing a non-existent element: {}", e.what());
} catch (const std::exception& e) {
// To catch any other standard library exceptions (they derive from std::exception)
std::println(stderr, "Exception thrown: {}", e.what());
} catch (...) {
// Catch any unrecognised exceptions (i.e. those which don't derive from std::exception)
std::println(stderr, "Some fatal error");
}
}
</syntaxhighlight>

It is also possible to raise exceptions purposefully, using the <syntaxhighlight lang="C++" inline>throw</syntaxhighlight> keyword; these exceptions are handled in the usual way. In some cases, exceptions cannot be used due to technical reasons. One such example is a critical component of an embedded system, where every operation must be guaranteed to complete within a specified amount of time. This cannot be determined with exceptions as no tools exist to determine the maximum time required for an exception to be handled.<ref>{{Cite book |title=The C++ Programming Language |last=Stroustrup |first=Bjarne |publisher=Addison Wesley |year=2013 |isbn=9780321563842 |pages=349}}</ref>

Unlike [[Signal handler|signal handling]], in which the handling function is called from the point of failure, exception handling exits the current scope before the catch block is entered, which may be located in the current function or any of the previous function calls currently on the stack.

== Enumerated types ==
{{excerpt|Enumerated type|C++}}

== Concepts ==
{{main|Concepts (C++)}}

== Code inclusion ==
=== Headers ===
{{see also|Include directive}}
Traditionally (prior to [[C++20]]), code inclusion in C++ followed the ways of C, in which code was imported into another file using the preprocessor directive <code>#include</code>, which would copy the contents of the file into the other file.

Traditionally, C++ code would be divided between a header file (typically with extension {{mono|.h}}, {{mono|.hpp}} or {{mono|.hh}}) and a source file (typically with extension {{mono|.cpp}} or {{mono|.cc}}). The header file usually contained declarations of symbols while the source file contained the actual implementation, such as function implementations. This separation was often enforced because <code>#include</code>ing code into another file would result in it being reprocessed for each file it was included by, resulting in increased compilation times if the compiler had to reprocess the same source repeatedly.

Headers often also forced the usage of [[include guard|{{mono|#include}} guards]] or [[pragma once|{{mono|#pragma once}}]] to prevent a header from potentially being included into a file multiple times.

The C++ standard library remains accessible through headers, however since C++23 it has been made accessible using modules as well.<ref name=cppreferencemodules /><ref name=cppreferencestandardlibrary /> Even with the introduction of modules, headers continue to play a role in modern C++, as existing codebases have not completely migrated to modules.

=== Modules ===
{{excerpt|Precompiled header|Modules}}

== See also ==
* [[C syntax]]
* [[Java syntax]]
* [[C Sharp syntax]]

== References ==
{{reflist|30em|refs=
<ref name=cppreferencekeywords>{{cite web |title=C++ keywords |url=https://en.cppreference.com/w/cpp/keyword |author=cppreference.com |year=2025 |access-date=2025-02-26}}</ref>
<ref name=cppreferencemodules>{{cite web |title=Modules (since C++20) |url=https://en.cppreference.com/w/cpp/language/modules |author=cppreference.com |year=2025 |access-date=2025-02-20}}</ref>
<ref name=cppreferencestandardlibrary>{{cite web |title=C++ Standard Library |url=https://en.cppreference.com/w/cpp/standard_library |author=cppreference.com |year=2025 |access-date=2025-02-20}}</ref>
}}

{{C++ programming language}}

[[Category:C++]]
[[Category:Source code]]
[[Category:Programming language syntax]]
<!-- Hidden categories below -->
[[Category:Articles with example C++ code]]

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